1. Technical Field
The present invention relates to a pressure sensor, and more particularly to a pressure sensor capable of suppressing measurement errors due to a change in temperature.
2. Related Art
In the related art, pressure sensors which use a piezoelectric vibrating element as a pressure sensitive element are known such as a hydraulic pressure meter, a barometer, a differential pressure meter, and the like. In a pressure sensor using a piezoelectric vibrating element, when pressure is applied to the piezoelectric vibrating element in the detection axis direction thereof, the resonance frequency of the piezoelectric vibrating element changes, and pressure applied to the pressure sensor is detected from a change in the resonance frequency.
JP-A-2007-57395, JP-A-2005-121628, and JP-A-2010-25582 disclose pressure sensors that use a piezoelectric vibrator as a pressure sensitive element. FIG. 13 shows a pressure sensor disclosed in JP-A-2007-57395. The pressure sensor 201 disclosed in JP-A-2007-57395 includes an airtight case 202 having first and second pressure inlet openings 203a and 204a respectively on first and second walls 203 and 204 facing each other, a first cylindrical bellows 210 of which one end is fixed to the first wall 203 and which has a shaft hole communicating with the first pressure inlet opening 203a, a second cylindrical bellows 211 of which one end is fixed to the second wall 204 and which has a shaft hole communicating with the second pressure inlet opening 204a and is disposed in series with the first bellows 210, a vibration element attachment pedestal 215 that is disposed between and fixed to the other ends of the first and second bellows 210 and 211, a thin plate-like piezoelectric vibrating element 220 that is supported by the vibration element attachment pedestal 215, and an oscillation circuit 230 that is electrically connected to an electrode pattern on the piezoelectric vibrating element 220. The piezoelectric vibrating element 220 has one end fixed to the second wall 204 and has the other end fixed to the vibration element attachment pedestal 215. A piezoelectric reinforcement plate 221 is fixed between the second wall 204 and the vibration element attachment pedestal 215 at a position facing the piezoelectric vibrating element 220 with the second bellows 211 disposed therebetween. The inner wall of the airtight case 202 and the vibration element attachment pedestal 215 are connected by an elastic reinforcement member 250. The same technique is disclosed in JP-A-2005-121628.
The pressure sensor 201 disclosed in JP-A-2007-57395 has a configuration in which the first and second bellows 210 and 211 are disposed in series or coaxially within a case maintained in a vacuum or inert-gas atmosphere. The pressure sensor 201 uses a principle in which the resonance frequency of the piezoelectric vibrating element 220 changes when a displacement force in the axial direction of the respective bellows generated by pressure applied to the shaft holes of the respective bellows is transmitted to the piezoelectric vibrating element 220 disposed in the airtight case 202. With this configuration, it is possible to realize a high-accuracy pressure sensor at a low cost without using an expensive electrodeposited bellows or a complex supporting structure. Moreover, since the elastic reinforcement member 250 is disposed between the vibration element attachment pedestal 215 and the inner wall of the airtight case 202, it is possible to increase the strength against impact from a direction vertical to the axial direction.
FIG. 14 shows a pressure sensor disclosed in JP-A-2010-25582. The pressure sensor 310 disclosed in JP-A-2010-25582 includes a cylindrical housing 312, first and second diaphragms 314 and 316 that respectively seal openings at both ends of the housing 312, a first reaction generating portion 320 that is connected to the first diaphragm 314 so as to apply force to the first diaphragm 314 in the opposite direction to the gravity received by the first diaphragm 314 using a first weight 328 by the principle of leverage, a second reaction generating portion 322 that is connected to the second diaphragm 316 so as to apply force to the second diaphragm 316 in the opposite direction to gravity received by the second diaphragm 316 using a second weight 330 by the principle of leverage, and a pressure sensitive element 318 which is disposed inside the housing 312, and in which a force detection direction is set as a detection axis thereof, one end thereof is connected to the first diaphragm 314, and the other end thereof is connected to the second weight 330.
In the above configuration, the pressure sensitive element 318 receives stress in a direction pushing it away from the first diaphragm 314 in response to the pressure received by the first diaphragm 314, whereas the pressure sensitive element 318 receives stress in a direction pulling it toward the second diaphragm 316 in response to the pressure received by the second diaphragm 316. However, when the pressure received by the first and second diaphragm 314 and 316 is the same, although the pressure sensitive element 318 is displaced, no load is applied to the pressure sensitive element 318. Therefore, it is possible to measure relative pressure detected from a pressure difference between the first and second diaphragms 314 and 316. Moreover, since the detection axis of the pressure sensitive element 318 and the displacement direction of the first and second diaphragms 314 and 316 are arranged coaxially, it is possible to accurately measure the pressure difference between the first and second diaphragms 314 and 316.
With the above configuration, the first reaction generating portion 320 always applies force in the opposite direction to a resultant force which is the sum of stress corresponding to bending deformation caused by the gravity received by the first diaphragm 314 and the gravity received by the pressure sensitive element 318. The second reaction generating portion 322 always applies a force in the opposite direction to a resultant force which is a subtraction of the gravity received by the pressure sensitive element 318 from stress corresponding to bending deformation caused by the gravity received by the second diaphragm 316. Therefore, the displacement caused by the gravity received by the first and second diaphragms 314 and 316 is canceled, and the stress applied to the pressure sensitive element 318 caused by the gravity received by the first and second diaphragms 314 and 316 is canceled. Accordingly, the pressure sensitive element 318 detects only the pressure difference between the first and second diaphragms 314 and 316. Thus, it is possible to obtain a pressure sensor 310 in which a change in application of gravitational acceleration and the influence of vibration generated by the change are suppressed.
However, when the temperature changes, thermal deformation is applied to the piezoelectric vibrating element 220 and the pressure sensitive element 318 due to a difference in thermal expansion coefficients between the piezoelectric vibrating element 220 and the airtight case 202 in JP-A-2007-57395 and between the pressure sensitive element 318 and the housing 312 in JP-A-2010-25582. As a result, the resonance frequency changes, and it is difficult to measure pressure accurately.
FIG. 15 shows a pressure sensor disclosed in JP-A-2010-48798. In order to solve the above problems, the pressure sensor 410 disclosed in JP-A-2010-48798 includes a housing 412, a diaphragm 424 which seals an opening 422 of the housing 412 and includes a flexible portion and a peripheral region 424c positioned on the outer side of the flexible portion, and in which one principal surface of the flexible portion is a pressure receiving surface, and a pressure sensitive element 440 which includes a pressure sensing portion and first and second base portions 440a and 440b respectively connected to both ends of the pressure sensing portion, and in which an arrangement direction of the first and second base portions 440a and 440b is parallel to a displacement direction of the diaphragm 424. In the pressure sensor 410, the first base portion 440a is connected to a central portion of the diaphragm 424, which is the rear side of the pressure receiving surface, and the second base portion 440b is connected to the peripheral region 424c on the rear side, or to an inner wall of the housing 412 facing the first base portion 440a, through a connecting member 442.
With this configuration, the first base portion 440a disposed at one end in the detection axis direction of the pressure sensitive element 440 is connected to the central portion of the diaphragm 424 which is displaced by pressure from the outside. The second base portion 440b disposed at the other end on the opposite side of the one end is connected to the peripheral region 424c of the diaphragm 424, which is fixed to the housing 412 and is not displaced by pressure from the outside, or to the inner wall of the housing 412 facing the first base portion 440a, through the connecting member 442. Therefore, the pressure sensor 410, in which the pressure sensitive element 440 receives compressive stress due to pressure from the outside, measures absolute pressure. Moreover, since both ends of the pressure sensitive element 440 are connected to the side of the diaphragm 424, it is possible to reduce pressure measurement errors accompanied by a change in temperature resulting from a difference in the linear expansion coefficients of the pressure sensitive element 440 and the housing 412 which are formed of different materials. Furthermore, by forming the pressure sensitive element 440 integrally with the connecting member 442 using a piezoelectric material, thermal deformation between the pressure sensitive element 440 and the connecting member 442 can be prevented. Thus, it is possible to reduce pressure measurement errors.
However, in the pressure sensor 410 of JP-A-2010-48798, it is possible to prevent the occurrence of thermal deformation in the detection axis direction of the pressure sensitive element 440. However, since the connecting member 442 and the diaphragm 424 are formed of different materials, thermal deformation occurs between the diaphragm 424 and a portion of the connecting member 442 extending in a direction vertical to the detection axis direction of the pressure sensitive element 440. Moreover, since the connecting member 442 receives the thermal deformation, the pressure sensitive element 440 receives the thermal deformation from the connecting member 442. Thus, it is not possible to sufficiently eliminate the effect of thermal deformation.